To eliminate clogging in various facilities (cement kilns, bulk storage silos, grain silos, etc.), instead of using tools requiring manual operation by a worker such as scaling bars, it is known to use air guns, each of which produces a blast of air that will destroy or disperse the mass of material that has accumulated since the previous blasting.
These air guns work in the following way:
Air is accumulated in a tank fed by a compressed air supply circuit; a main valve controlled by a secondary valve controls the very rapid release of the air accumulated in the tank.
The very rapid discharge of the pressurized air contained in the tank produces a blast.
In the known air guns, the placement of the main valve is such that, locally, the front face of the valve piston is subjected to the pressure prevailing inside the tank, while the rear face of said piston is subjected to an equal counter-pressure prevailing in a rear chamber, itself fed by the tank.
Given the differences between the front and rear surfaces of the main valve subjected to equal pressure, the main valve is held in its seat by a slight force differential.
In order to open the main valve, a secondary solenoid-type valve triggers the evacuation of the rear chamber via a discharge conduit.
The evacuation of the air contained in the rear chamber creates an imbalance in the forces exerted on the piston of the main valve, allowing the opening that enables the air to be abruptly discharged from the tank because of the rapid movement of the piston.
To open the rear chamber, it suffices to supply electric power to the electromagnet of a solenoid valve so that the main valve opens. Since the volume of the chamber is small, this discharge is rapid.
The control module of the secondary solenoid-type valve is often mounted directly on the air gun, and this control module is supplied with power by an electric power cable. The control module uses a timer or counter that makes it possible to adjust the time between two blasts. The electric cable supplies power to the control module.
These devices are widely used in cement production facilities in which the environmental conditions are brutal. These materials are produced in kilns in an environment containing corrosive dust with a temperature of more than 50° C.
The advantage of these devices is that they can operate automatically; in some cases, they are positioned in places that are difficult to access and/or in areas where the temperature and/or dust conditions are dangerous.
They make it possible to avoid endangering operators working manually with scaling bars.
Several air guns are often necessary to handle a single area.
Generally, they work in complementary fashion, meaning that several air guns installed in one area will not all be actuated at the same time, but in a cycle that must be adjusted in accordance with various parameters.
The operating cycle is determined in advance, but sometimes changes are made during use. As mentioned, the firing rate is adjusted by programming a unit located on the rear of the air gun or in its immediate proximity, or in a remote control box housing a command generator. In theory, this programming is done during the installation of the air gun, since any subsequent intervention will be difficult, especially if it requires someone to work in proximity to the gun.
Electric power is supplied to the control module by an electric cable running through the structures of the plant, which in theory allows it to operate continuously. Unless there is an accidental cutoff of the power supply, the control module is always powered up. These cables are supported by cable trays, which in theory should not pose a problem, but because they are in a hostile environment, the presence of these cables increases the risk of failures, for example resulting from insufficient insulation or the accidental severing of a cable (the length of the cables often exceeds 100 meters). It is increasingly common for the actual control unit of the solenoid valve to be remote from the gun, so there is also a control cable running through the cable trays. In that case, the command generator transmits a command via a wired connection to the control module, which actuates either the secondary valve or an actuator.
Because of the difficulty of access, users only rarely intervene between maintenance operations and do not attempt to optimize the firing rates.
Thus, if the working conditions change, causing more accumulations to be generated, the system is left to run with no modifications until there is a shutdown of the facility or an urgent need.
Periodically, it is necessary to inspect the operation of the air gun, and thus the module is equipped with a manual control placed on the gun. This requires the technician to access the gun and fire it. Since these inspections take place in dangerous locations, there is a long interval between inspections.
It must not be forgotten that the blast produces a significant level of noise, especially if the operator is nearby.
The air tank is supplied by an air conduit and the electronic control unit is supplied with power by an electric cable. These supply lines must obviously be connected to the structures via different cable trays.
In short, the air gun, its tank, and its electronic control unit are positioned as near as possible to the area to be treated, and this assembly is supplied by both an air conduit and an electric cable. Assuming that the power supply is “endless,” and that the electric cable is a physical connection, a properly equipped installation should not have any supply problems.
Unfortunately, it has been observed that malfunctions can occur due to severed cables or an error in the control unit that has gone unnoticed. Thus, personnel are regularly sent in proximity to the gun to fire a blast and/or to verify that the gun is firing at regular intervals.
It is clear that this is dangerous.
Installations using several guns are provided with a main control box that sends instructions to secondary control boxes which control a given number of guns. There is therefore cabling (wired connections) between the main box and the secondary boxes, and between the secondary boxes and the air guns.
Since all of the cables pass through the wall of the control box, each passage is provided with a mandatory cable gland due to the corrosive and dusty environment (risk of explosion).
The wired connection between the command generator and the control module theoretically makes it possible to carry a signal with long-term reliability; however, it increases the risk of faulty instructions due to a rupture in the cables carrying the signal. It must not be forgotten that this is a dangerous industrial environment with long distances. Likewise, because of losses, the distance between the gun and the command generator is limited to around 50 meters. Thus, in industries in which air guns are spread over a large surface area, the number of command generators must be increased and a monitoring technician must move from one generator to another to make sure that there have been no malfunctions.
In a conventional (wired) installation, once the command is sent to the gun via the wired communication line, the blast is considered to have taken place even if a failure occurs. In reality, the solenoid valve may remain open or closed, in which case the blast does not occur. Often the malfunction does not become evident until it is too late.
There is an installation known from WO 2006/096092, which describes a clog sensor connected to a control module—itself connected to an air gun—which will indicate the presence of an accumulation and trigger a blast. That document indicates the theoretical possibility of replacing the cables with a wireless network without providing any details on its design.
Wireless transmission is a known means, but because it poses problems in industrial environments in which there are many metal parts that interfere with the delivery of the signals, it is not used.